The present invention generally relates to a biosensor and more particularly, to a biosensor which is provided, on a foundation electrode, with an active film having physiologically active material fixed thereonto, so as to generate an electric signal corresponding to an object material to be measured on the basis of the result of the activation reaction in the foundation electrode.
Since such characteristics of the physiologically active material have been noticed that it can detect considerably complicated organic compounds, protein or the like with high sensitivity and selectively, studies have been conducted with the use of the physiologically active material, in other words, by forming a biosensor which has the physiologically active material fixed onto the surface of a foundation electrode, thereby to measure the organic compounds, protein, etc.
When the objective material to be measured is measured by the use of the above-described biosensor, the oxidation and reduction or the like of the objective material have been generally carried out in the presence of the physiologically active material, so that the amount of the formed material or the lost material has been measured, thereby to measure the density of the objective material. The aforementioned foundation electrode is roughly classified into two types, namely, the two-electrode type as shown in FIG. 11 and three-electrode type as shown in FIGS. 12 and 13.
More specifically, the two-electrode type of FIG. 11 has a working electrode 31 made of platinum group metal or the like and a counter electrode 32 made of silver or the like which is spaced a predetermined distance from the working electrode 31. It is so arranged according to this type of the foundation electrode that a current signal is generated corresponding to the amount of the material formed or lost as the result of the reaction between the two electrodes.
Accordingly, the density of the objective material can be measured on the basis of the thus-obtained current signal.
In the above-described type of the foundation electrode, however, it is difficult to stabilize the potential between both the electrodes. And, it is general that the potential between the two electrodes is tried to be stabilized by making the area of the counter electrode 32 sufficiently large with respect to the working electrode 31, resulting in bulky structure of the foundation electrode as a whole. Moreover, the counter electrode is generally made of a precious metal, and therefore the foundation electrode itself becomes expensive in accordance with the increase of the size or the area of the precious counter electrode.
On the other hand, according to the threeelectrode type shown in FIGS. 12 and 13, the counter electrode 32 and a reference electrode 33 are placed in symmetry with respect to the working electrode 31, and a bias voltage corresponding to the change of the potential of the counter electrode 32 is applied between the counter electrode 32 and the reference electrode 33 by a direct current source 34 and an operational amplifier 35.
Accordingly, even when the counter electrode 32 has not so large an area, the potential stability can be enhanced, and consequently, the density of the object material is able to be measured with remarkable accuracy on the basis of the obtained current signal.
However, the density of the objective material can not be measured correctly under every condition. For example, so long as the density of the objective material is measured using the balanced current, that is, the measurement is stationary and regular, the density of the objective material is correctly measured. On the contrary, when the measurement of the density of the objective material is carried out on the basis of the change rate of the current, the potential follow-up property of the reference electrode 33 gives great influences upon the measurement accuracy. Accordingly, it becomes necessary that the reference electrode 33 and the counter electrode 32 satisfy the same relationship as the working electrode 31 and the counter electrode 32 of the earlier described two-electrode type, requiring a bias supply circuit for the reference electrode 33. As a result, the structure of the foundation electrode is unfavorably complicated. This will be described in more detail. When the density of the objective material is measured by the above-described biosensor, in general, it is measured by measuring the amount of material produced or lost as a result of the reaction such as oxidation, reduction, etc. of the objective material in the presence of the physiologically active material. Therefore, the measurable upper limit of the density is dependent on the amount of the material causing the oxidation, reduction, etc., for example, oxygen present in the objective material. Thus, even when the density of the object is measured on the basis of the balanced current, the density of the object, if it is highly dense, can never be detected accurately. For solving such disadvantage as above, it may be considered that the density is measured on the basis of the current primary differential peak value. In this case, however, the follow-up property of the reference electrode 33 is required to be increased so as to obtain accurate current primary differential value. Otherwise, correct density of the objective material can not be measured.